(a) Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly it pertains to a digital semiconductor integrated circuit device which is of the type that a dynamic MOS circuit block which is comprised of a combination of such transistors as MOS transistors, and a static bipolar circuit block which is comprised of a combination of such transistors as bipolar transistors, are arranged in a common semiconductor substrate. Hereinafter, the term "integrated circuit" will be used in its abbreviated form "IC" throughout this specification.
(b) Description of the Prior Art
The semiconductor integrated circuits which have been called by such well-known abbreviated terms as IC, LSI and VLSI are divided roughly into the two large groups, analog IC and digital IC.
The former are employed in electronic apparatuses for household use or for tele-communication, which deal with analog electric signals wherein the waveshape per se of an input electric signal such as an audio signal provides for significant information. The latter, on the other hand, are employed in such electronic apparatuses that deal with digital electric signals as are used typically in electronic computers.
The digital IC may also be roughly divided further into logic IC and memory IC when viewed from the point of their operating functions, although there are many cases in which a memory IC having a memorizing function is included in the sphere of a logic IC.
There exists an IC having a single function such that it has either the "memory" function or the logic function. Many of the ICs are such that there are incorporated, in a single semiconductor chip or slice, a number of peripheral circuits having other functions such as pulse generation, power supply and buffering, in addition to said memory or logic functions.
In any one of the digital ICs such as a memory IC and a logic IC, it is well known that the principal constituting elements which form the circuit blocks having those functions as mentioned above are transistors. At present, transistors are roughly divided into bipolar transistors and unipolar transistor (i.e. MOS-FET). Therefore, devices are called either MOS IC or bipolar IC, depending on the difference in the type of the transistors employed. For example, a typical digital IC using a bipolar transistor includes TTL (Transistor-Transistor-Logic) and IIL (Integrated-Injection-Logic).
It is added here that a transistor called a "Static Induction Transistor" has been recently developed, and its superiority in characteristics when it is incorporated in an IC has been found to be prominently useful. Thus, various semiconductor devices will have to be re-classified soon. However, description in this specification will be made by following the manner of grouping of transistors which has been accepted in the past. Regarding this new type of transistor, i.e. SIT, reference will be made later in this specification.
Now, when the abilities or functions of a digital IC is studied, attention is paid to such points as the speed of logic operation, or the operating velocity of, for example, access time or cycle time, for memory as well as power dissipation, packing density, easiness in laying out various circuit blocks in a semiconductor substrate, easiness of fabrication, and price.
Although, owing to the recent remarkable technical progress in the field of semiconductor devices, the functions and abilities of MOS ICs have approached quite close to those of bipolar ICs, making it difficult to discriminate which is superior to the other, there still is the difference in functions and abilities attributable to the difference in the operation principles of MOS transistors and bipolar transistors. Such difference may be briefly described as follows.
A bipolar IC, as compared with a MOS IC, is quicker in the velocity of performing memory operation, and this velocity would not change so prominently even by a change in the manner of layout of, for example, the constituent transistors employed. In other words, a bipolar IC is such that its operation velocity has little dependency on layout. Also, a bipolar IC is such that its circuit arrangement is made in the usual manner, i.e. in static type, which basically uses six (6) transistors as a circuit arrangement to form an one bit memory. In contrast thereto, a MOS IC not only is able to form the abovesaid static type circuit arrangement, but also allows one to form a dynamic type circuit arrangement which utilizes the lengthy period of time required for the electric charge stored in the gate region of the MOS transistor to discharge. For example, the circuit arrangement for forming an one bit memory requires, in the simplest form, one transistor and one capacitor. Therefore, the area required of a chip can be considerably saved, and thus, a MOS IC has the advantage over a bipolar IC in that the former is capable of providing a much higher packing density. The operation velocity of a MOS IC, however, not only is inferior to a bipolar IC as stated above, but also it considerably depends on the layout of, for example, the transistors which are incorporated. Thus, the MOS IC has a large dependency on layout.
Taking up, for example, power dissipation and price, a MOS IC is indeed superior to a bipolar IC, but a bipolar IC is easier to handle as compared with a MOS IC with respect to a circuit connection with a bipolar IC, or with respect to matching to a power supply.
As stated above, a MOS IC and a bipolar IC have a difference in their functions and abilities, and thus there has been a demand for the development of such an IC that can effectively make use of these respective strong points of these two types of ICs.